Project Summary/Abstract This project aims to investigate the circuit mechanisms that enable learned complex behaviors, such as playing the violin or hitting a tennis forehand. At present, the processes which underlie the generation of these behavioral sequences by neural networks are poorly understood. Specifically, the contributions of various cell types to this network behavior remain underexplored. Here we consider an important motor control nucleus in the zebra finch called HVC (formerly known as the high vocal center), which produces neural sequences during the performance of the learned courtship song. Our proposal focuses on local circuit interneurons, which represent the sole source of inhibition to this network. Several models have been proposed to explain the role of inhibition in song production, but consensus remains elusive. To test these models, we propose a series of electrophysiological, imaging, and optogenetic studies that will enable us to manipulate and monitor HVC interneurons selectively, often in the context of song production. In Aim 1, we will examine the role of inhibition from a postsynaptic perspective. The primary focus of this aim is to record inhibitory synaptic currents onto HVC projection neurons during song production. In Aim 2, we will examine the role of inhibition from a presynaptic perspective by directly measuring populations of identified interneurons during singing. We will also gauge the impact of individual interneurons on the network using anatomical and electrophysiological methods. In Aim 3, we will use in vivo and in vitro measurements to characterize the genetic subtypes of HVC interneurons and to distinguish their roles within the network.